A piezoelectric actuator comprises a stack of laminated sheets or layers of piezoelectric ceramic elements. The piezoelectric ceramic elements or electrode thereof are arranged in an alternating fashion wherein alternating electrode portions are disposed at either side of the stack. In order to actuate the stack of piezoelectric material, electrical energy is distributed within the stack via a common electrode for each polarity. A stack of piezoelectric material has many internal electrode layers of alternating polarity. These alternating layers require a reliable electrical connection to a common electrode electrically terminated therewith. The common electrode, one on each side of the stack, will distribute the voltage in order to actuate the stack.
During stack actuation, the stack itself will expand and contract thus, piezoelectric actuators are used for actuating control valves or fuel injection valves in vehicles as they can be designed to provide precise ranges of actuation when subject to an actuating voltage. Accordingly, the common electrodes disposed at either side of the stack must be capable of many cycles of elastic elongation and contraction without breaking the electrical connection.
Conductive polymers or metal electrodes that have complex multiple connections to the stack, are currently used for the common electrode function. The polymers have elastic properties that allow the electrodes to move with the piezoelectric, material at high cycle rates with strains of approximately 0.15%. Suitable conductive polymers have metal particles that randomly touch each other to allow for electrical current flow. However, and due to the usage of metal particles, the conductive polymers will have lower conductance per unit area than wrought metal electrodes. Accordingly, connection points or small section areas can result in localized overheating because of the limited number of metal particles contacting each other. In contrast, wrought metal electrodes are generally excellent conductors as they can be connected with low contact resistance using common connection technologies like soldering or spot welding. They also have properties that excel at the high frequency or current changes that are used in piezoelectric stacks. In addition, these metal electrodes produce a minimum resistance heating at operating conditions typically encountered by piezoelectric stacks.
However, the major drawbacks for using wrought metal side electrodes in piezoelectric actuators can be twofold: i) simple designs are not able to handle the cyclic strain of stack actuation without cracking, and ii) complex designs can handle the strain, but have multiple attachment points and/or surfaces bonded to the stack.
Therefore, it is desirable to provide a simplified strain tolerant metal side electrode with a single attachment surface for use with a piezoelectric actuator.